Conventional light emitting devices, such as incandescent, halogen and fluorescent lamps, have not been significantly improved in the past twenty years. However, light emitting diode (“LEDs”) have been improved to a point with respect to operating efficiency where LEDs are now replacing the conventional light emitting devices in traditional monochrome lighting applications, such as traffic signal lights and automotive taillights. This is due in part to the fact that LEDs have many advantages over conventional light emitting devices. These advantages include longer operating life, lower power consumption, and smaller size.
LEDs use semiconductor LED dies as light sources, which typically produce monochromatic light. Currently, LEDs are available in various colors from UV-blue to green, yellow and red. Due to the narrow-band emission characteristics of the LED dies, these LEDs cannot be directly used for “white” light applications. Rather, the output light of a monochromatic LED die must be mixed with other light of one or more different wavelengths to produce white light. Two common approaches for producing white light include (1) packaging individual red, green and blue LED dies together so that light emitted from these LED dies are combined to produce white light and (2) introducing a photoluminescent material into a UV, blue or green LED so that some or all of the original light emitted by the semiconductor die of the LED is converted into longer wavelength light and combined with the original blue or green light to produce white light.
Between these two approaches for producing white light using monochromatic LED dies, the second approach is generally preferred over the first approach. In contrast to the second approach, the first approach requires a more complex driving circuitry since the red, green and blue LED dies have different operating voltage requirements. Furthermore, failure to properly mix the light emitted from the different LED dies may result in output light with uneven color. In addition, since different types of semiconductor LED dies are used, the resulting mixed light is subject to variations due to the different operating characteristics of these LED dies with respect to temperature, chronological change and operating environment.
A concern with the second approach is that the resulting white light may not have a high color-rendering index, which is required for certain backlighting and illumination applications. As an example, a white LED that uses a blue LED die with YAG:Ce phosphor will produce white light that is deficient in the red wavelength region.
In view of the above concerns, there is a need for a device and method for emitting white output light having a high color-rendering index.